3A, 17V Current Mode Synchronous Step-Down Converter...3A, 17V Current Mode Synchronous Step-Down...
Transcript of 3A, 17V Current Mode Synchronous Step-Down Converter...3A, 17V Current Mode Synchronous Step-Down...
RT7296B
Copyright © 2018 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
DS7296B-03 May 2018 www.richtek.com 1
3A, 17V Current Mode Synchronous Step-Down Converter
General Description The RT7296B is a high-efficiency, 3A current mode
synchronous step-down DC-DC converter with a wide
input voltage range from 4.5V to 17V. The device
integrates 80m high-side and 30m low-side
MOSFETs to achieve high efficiency conversion. The
current mode control architecture supports fast
transient response and internal compensation. A
cycle-by-cycle current limit function provides protection
against shorted output. The RT7296B provides
complete protection functions such as input
under-voltage lockout, output under-voltage protection,
over-current protection, and thermal shutdown. The
PWM frequency is adjustable by the EN/SYNC pin. The
RT7296B is available in the TSOT-23-8 (FC) package.
Ordering Information
Package Type
J8F : TSOT-23-8 (FC)
Lead Plating System
G : Green (Halogen Free and Pb Free)
RT7296B
Note :
Richtek products are :
RoHS compliant and compatible with the current
requirements of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
Features 4.5V to 17V Input Voltage Range
3A Output Current
Internal N-Channel MOSFETs
Current Mode Control
Fixed Switching Frequency : 500kHz
Synchronous to External Clock : 200kHz to 2MHz
Cycle-by-Cycle Current Limit
External Soft-Start Function
Input Under-Voltage Lockout
Output Under-Voltage Protection
Thermal Shutdown
Applications
Industrial and Commercial Low Power Systems
Computer Peripherals
LCD Monitors and TVs
Set-top Boxes
Marking Information
01=DNN01= : Product Code
DNN : Date Code
Simplified Application Circuit
Enable
VIN
EN/SYNC
GND
BOOT
FB
SW VOUT
VIN
RT7296B
R5 R1
R2
PVCC
SS
C3
C4C2
L1
C5
C1
RT7296B
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Pin Configuration (TOP VIEW)
SS
SW
GN
D
FB
EN
/SY
NC
BO
OT
VIN
PV
CC
5
3 4
68
2
7
TSOT-23-8 (FC)
Functional Pin Description
Pin No. Pin Name Pin Function
1 SS Soft-start control input. SS control the soft-start period. Connect a capacitor
from SS to GND to set the soft-start period.
2 VIN Power input. Support 4.5V to17V Input Voltage. Must bypass with a ceramic
capacitor at this pin.
3 SW Switch node. Connect to external L-C filter.
4 GND System ground.
5 BOOT Bootstrap supply for high-side gate driver. Connect a 0.1F ceramic capacitor
between the BOOT and SW pins.
6 EN/SYNC
Enable control input. High = Enable. Apply an external clock to adjust the
switching frequency. If using pull high resistor connected to VIN, the
recommended value range is 60k to 300k.
7 PVCC 5V bias supply output. Connect a minimum of 0.1F capacitor to ground.
8 FB
Feedback voltage input. The pin is used to set the output voltage of the
converter to regulate to the desired voltage via a resistive divider. Feedback
reference = 0.807V.
RT7296B
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Functional Block Diagram
EN/SYNC
+
- UV Comparator
Oscillator
0.4V
Internal
Regulator
Shutdown
Comparator BOOT
GNDFB
HS Switch
Current
Comparator
11µA
Power
Stage &
Deadtime
Control
+
-
1.4V
Slope
Compensation
LS Switch
Current
Comparator
UVLO
Logic &
Protection
Control
BOOT
UVLO
Current
Sense
Current
Sense
PVCC
SS
50pF
1pF
400k
SW
VIN
0.807V+
-EA+
Operation
Under Voltage Lockout Threshold
The IC includes an input Under Voltage Lockout
Protection (UVLO). If the input voltage exceeds the
UVLO rising threshold voltage (3.9V), the converter
resets and prepares the PWM for operation. If the input
voltage falls below the UVLO falling threshold voltage
(3.25V) during normal operation, the device stops
switching. The UVLO rising and falling threshold
voltage includes a hysteresis to prevent noise caused
reset.
Chip Enable
The EN pin is the chip enable input. Pulling the EN pin
low (<1.1V) will disable the device. During shutdown
mode (<0.4V), the RT7296B’s quiescent current drops
to lower than 1A. Driving the EN pin high (>1.6V) will
turn on the device.
Operating Frequency and Synchronization
The internal oscillator runs at 500kHz (typ.) when the
EN/SYNC pin is at logic-high level (>1.6V). If the EN
pin is pulled to low-level over 8s, the IC will shut down.
The RT7296B can be synchronized with an external
clock ranging from 200kHz to 2MHz applied to the
EN/SYNC pin. The external clock duty cycle must be
from 20% to 80% with logic-high level = 2V and
logic-low level = 0.8V.
Internal Regulator
The internal regulator generates 5V power and drive
internal circuit. When VIN is below 5V, PVCC will drop
with VIN. A capacitor(>0.1F) between PVCC and
GND is required.
External Soft-Start Function
The RT7296B provides external soft-start function. The
soft-start function is used to prevent large inrush
current while converter is being powered-up. The
soft-start timing can be programmed by the external
capacitor between SS pin and GND. The Chip provides
a 11A charge current for the external capacitor 100s
after EN rising. As shown in Figure 1, output voltage
starts to rise after 0.4 x tSS.
RT7296B
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VOUT
SS
EN
VIN
VCC
VIN = 12V
VCC = 5V
tSS0.4 x tSS
0.1ms
Figure. 1 Start-Up Sequence
High-Side MOSFET Over-Current Limit
The RT7296B features cycle-by-cycle current-limit
protection and prevents the device from the
catastrophic damage in output short circuit, over
current or inductor saturation. During the on-time of the
high side switch, the device monitors the switch current.
If the switch current overs the current limit threshold,
the device turns off the high side switch to prevent the
device from damage.
Output Under-Voltage Protection
The RT7296B includes output under-voltage protection
(UVP) against over-load or short-circuited condition by
constantly monitoring the feedback voltage VFB. If VFB
drops below the under-voltage protection trip threshold,
50% (typ.) of the internal reference voltage, the UV
comparator will go high to turn off the internal high-side
MOSFET switches. If the output under-voltage
condition continues for a period of time, the RT7296B
will enter output under-voltage protection with hiccup
mode. During hiccup mode, the device remains shut
down. After a period of time, a soft-start sequence for
auto-recovery will be initiated. Upon completion of the
soft-start sequence, if the fault condition is removed,
the converter will resume normal operation; otherwise,
such cycle for auto-recovery will be repeated until the
fault condition is cleared. Hiccup mode allows the
circuit to operate safely with low input current and
power dissipation, and then resume normal operation
as soon as the over-load or short-circuit condition is
removed. The UVP profile is shown in Figure 2.
Thermal Shutdown
Thermal shutdown is implemented to prevent the chip
from operating at excessively high temperatures. When
the junction temperature is higher than 150oC, the chip
will shutdown the switching operation. The chip is
automatically re-enabled when the junction temperature
cools down by approximately 20oC.
Vout
Abnormal case detected (UV)
1.8 x tSS
SW
tSS / 3
Figure. 2 Output Under-Voltage Protection with Hiccup Mode
RT7296B
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Absolute Maximum Ratings (Note 1)
Supply Input Voltage, VIN ----------------------------------------------------------------------------------- 0.3V to 20V
Switch Voltage, SW -------------------------------------------------------------------------------------------- 0.3V to VIN + 0.3V
<20ns --------------------------------------------------------------------------------------------------------------- 5V
BOOT to SW, VBOOT – SW ----------------------------------------------------------------------------------- 0.3V to 6V (7V for < 10s)
Bias Supply Output, PVCC---------------------------------------------------------------------------------- 0.3V to 6V (7V for < 10s)
Other Pins--------------------------------------------------------------------------------------------------------- 0.3V to 6V
Power Dissipation, PD @ TA = 25C
TSOT-23-8 (FC) ------------------------------------------------------------------------------------------------ 1.428W
Package Thermal Resistance (Note 2)
TSOT-23-8 (FC), JA ----------------------------------------------------------------------------------------- 70C/W
TSOT-23-8 (FC), JC ----------------------------------------------------------------------------------------- 15C/W
Lead Temperature (Soldering, 10 sec.) ----------------------------------------------------------------- 260C
Junction Temperature ----------------------------------------------------------------------------------------- 40C to 150C
Storage Temperature Range ------------------------------------------------------------------------------- 65C to 150C
ESD Susceptibility (Note 3)
HBM (Human Body Model) --------------------------------------------------------------------------------- 2kV
Recommended Operating Conditions (Note 4)
Supply Input Voltage, VIN ----------------------------------------------------------------------------- 4.5V to 17V
Junction Temperature Range ------------------------------------------------------------------------------ 40C to 125C
Ambient Temperature Range ------------------------------------------------------------------------------ 40C to 85C
Electrical Characteristics (VIN = 12V, TA = 25C, unless otherwise specified)
Parameter Symbol Test Conditions Min Typ Max Unit
Shutdown Supply Current VEN = 0V -- -- 1 A
Quiescent Current with no Load
at DCDC Output VEN = 2V, VFB = 1V -- 0.8 1 mA
Feedback Voltage VFB 0.799 0.807 0.815 V
Feedback Current IFB VFB = 820mV -- 10 50 nA
Switch
On-Resistance
High-Side RDS(ON)H -- 80 -- m
Low-Side RDS(ON)L -- 30 --
Switch Leakage VEN = 0V, VSW = 0V -- -- 1 A
Current Limit ILIM Under 40% duty-cycle 4.2 5 5.8 A
Low-Side Switch Current Limit From drain to source -- 2 -- A
Oscillation Frequency fOSC VFB = 0.75V 440 500 580 kHz
SYNC Frequency Range fSYNC 200 -- 2000 kHz
Fold-Back Frequency VFB < 400mV -- 125 -- kHz
Maximum Duty-Cycle DMAX VFB = 0.7V 90 95 -- %
RT7296B
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Parameter Symbol Test Conditions Min Typ Max Unit
Minimum On-Time tON -- 60 -- ns
EN Input Voltage Logic-High VIH 1.2 1.4 1.6
V Logic-Low VIL 1.1 1.25 1.4
EN Input Current IEN VEN = 2V -- 2 --
A VEN = 0V -- 0 --
EN Turn-off Delay ENtd-off -- 8 -- s
Input Under-Voltage
Lockout Threshold
VIN Rising VUVLO VIN rising 3.7 3.9 4.1 V
Hysteresis VUVLO -- 650 -- mV
VCC Regulator VCC -- 5 -- V
VCC Load Regulation VLOAD IVCC = 5mA -- 3 -- %
Soft-Start Charge Current ISS -- 11 -- A
Thermal Shutdown Temperature TSD -- 150 -- oC
Thermal Shutdown Hysteresis TSD -- 20 -- oC
Note 1. Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These
are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in
the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may affect
device reliability.
Note 2. JA is measured at TA = 25C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. JC is
measured at the exposed pad of the package.
Note 3. Devices are ESD sensitive. Handling precaution recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
RT7296B
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Typical Application Circuit
VIN
EN/SYNC
GND
BOOT
FB
SW VOUT
VIN4.5V to 17V
RT7296B
Enable
R533k
R140.2k
R213k
PVCC
SS
C30.1μF
C444μF
C1
22μF
C20.1μF
L14.7μH
C522nF
R610
2
1
3
4
5
6
7
8
CFF 15pF
Table 1. Suggested Component Values
VOUT (V) R1 (k) R2 (k) R5 (k) CFF (pF) C4 (F) L1 (H)
1.0 20.5 84.5 82 15 44 2.2
3.3 40.2 13 33 15 44 4.7
5.0 40.2 7.68 33 15 44 4.7
Note : Where the C4 value means the effective output capacitance. Design engineer must be aware that ceramic
capacitance varies a great deal with the size, operating voltage and temperature. The variation should be taken into
the design consideration of control loop bandwidth. A rule-of-the-thumb is to design the RT7296B control loop
bandwidth below 60kHz by changing the value of R5. Generally, increase the value of R5 if a de-rated capacitance
is used.
RT7296B
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Typical Operating Characteristics
Efficiency vs. Output Current
0
10
20
30
40
50
60
70
80
90
100
0 0.5 1 1.5 2 2.5 3
Output Current (A)
Effic
ien
cy (
%)
VIN = 4.5V
VIN = 12V
VIN = 17V
VOUT = 1V
Efficiency vs. Output Current
0
10
20
30
40
50
60
70
80
90
100
0 0.5 1 1.5 2 2.5 3
Output Current (A)
Effic
ien
cy(%
)
VOUT = 3.3V
VIN = 4.5V
VIN = 12V
VIN = 17V
Efficiency vs. Output Current
0
10
20
30
40
50
60
70
80
90
100
0 0.5 1 1.5 2 2.5 3
Output Current (A)
Effic
ien
cy (
%)
VOUT = 5V
VIN = 7V
VIN = 12V
VIN = 17V
Output Voltage vs. Input Voltage
3.14
3.18
3.22
3.26
3.30
3.34
3.38
3.42
3.46
4 5 6 7 8 9 10 11 12 13 14 15 16 17
Input Voltage (V)
Ou
tpu
t V
olta
ge
(V
)
VOUT = 3.3V, IOUT = 3A
Reference Voltage vs. Temperature
0.76
0.77
0.78
0.79
0.80
0.81
0.82
0.83
0.84
-50 -25 0 25 50 75 100 125
Temperature (°C)
Re
fere
nce
Vo
lta
ge
(V
)
IOUT = 1A
Output Voltage vs. Output Current
3.14
3.18
3.22
3.26
3.30
3.34
3.38
3.42
3.46
0 0.5 1 1.5 2 2.5 3
Output Current (A)
Ou
tpu
t V
olta
ge
(V
)
VIN = 12V, VOUT = 3.3V
RT7296B
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UVLO Voltage vs. Temperature
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
-50 -25 0 25 50 75 100 125
Temperature (°C)
UV
LO
Vo
lta
ge
(V
)
Rising
Falling
VOUT = 3.3V, IOUT = 0A
EN Threshold vs. Temperature
1.15
1.20
1.25
1.30
1.35
1.40
1.45
1.50
-50 -25 0 25 50 75 100 125
Temperature (°C)
EN
Th
resh
old
(V
)
Rising
Falling
VOUT = 3.3V, IOUT = 0A
VIN = 12V, VOUT = 3.3V, L = 4.7H,
IOUT = 1.5A to 3A to 1.5A
VOUT
(100mV/Div)
IOUT
(1A/Div)
Time (200s/Div)
Load Transient Response
VIN = 12V, VOUT = 3.3V,
L = 4.7H, IOUT = 3A
VOUT
(20mV/Div)
VSW
(5V/Div)
Time (2s/Div)
Output Ripple Voltage
VIN = 12V, VOUT = 3.3V, IOUT = 3A
VOUT
(2V/Div)
VEN
(2V/Div)
VSW
(10V/Div)
IL(3A/Div)
Time (2ms/Div)
Power On from EN
VIN = 12V, VOUT = 3.3V, IOUT = 3AVOUT
(2V/Div)
VEN
(2V/Div)
VSW
(10V/Div)
IL(3A/Div)
Time (2ms/Div)
Power Off from EN
RT7296B
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VIN = 12V, VOUT = 3.3V, IOUT = 3A
VOUT
(2V/Div)
VIN
(10V/Div)
VSW
(10V/Div)
IL(3A/Div)
Time (5ms/Div)
Power On from VIN
VIN = 12V, VOUT = 3.3V, IOUT = 3AVOUT
(2V/Div)
VIN
(10V/Div)
VSW
(10V/Div)
IL(3A/Div)
Time (5ms/Div)
Power Off from VIN
RT7296B
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Application Information
The RT7296B is a high voltage buck converter that can
support the input voltage range from 4.5V to 17V and
the input voltage range from 4.5V to 17V and the output
current can be up to 3A.
Output Voltage Selection
The resistive voltage divider allows the FB pin to sense
a fraction of the output voltage as shown in Figure 3.
RT7296B
GND
FBR1
R2
VOUT
R5
Figure 3. Output Voltage Setting
For adjustable voltage mode, the output voltage is set
by an external resistive voltage divider according to the
following equation :
OUT FBR1
V V 1R2
Where VFB is the feedback reference voltage (0.807V
typ.). Table 1 lists the recommended resistors value for
common output voltages.
Table 2. Recommended Resistors Value
VOUT (V) R1 (k) R2 (k) R5 (k)
1.0 20.5 84.5 82
3.3 40.2 13 33
5.0 40.2 7.68 33
External Bootstrap Diode
Connect a 100nF low ESR ceramic capacitor between
the BOOT pin and SW pin. This capacitor provides the
gate driver voltage for the high side MOSFET. It is
recommended to add an external bootstrap diode
between an external 5V and BOOT pin, as shown as
Figure 4, for efficiency improvement when input voltage
is lower than 5.5V or duty ratio is higher than 65% .The
bootstrap diode can be a low cost one such as IN4148
or BAT54. The external 5V can be a 5V fixed input from
system or a 5V output (PVCC) of the RT7296B.
SW
BOOT
5V
RT7296B 100nF
Figure 4. External Bootstrap Diode
External Soft-Start Capacitor
The RT7296B provides external soft-start function. The
soft-start function is used to prevent large inrush
current while converter is being powered-up. The
soft-start timing can be programmed by the external
capacitor (CSS) between SS pin and GND. The Chip
provides a 11A charge current (ISS) for the external
capacitor. The soft-start time (tSS, VREF is from 0V to
0.8V) can be calculated by the following formula :
SSSS
SS
C (nF) 1.3t (ms) =
I ( A)
Inductor Selection
The inductor value and operating frequency determine
the ripple current according to a specific input and
output voltage. The ripple current IL increases with
higher VIN and decreases with higher inductance.
OUT OUTL
IN
V VI 1
f L V
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output
voltage ripple. High frequency with small ripple current
can achieve highest efficiency operation. However, it
requires a large inductor to achieve this goal.
For the ripple current selection, the value of IL =
0.3(IMAX) will be a reasonable starting point. The
largest ripple current occurs at the highest VIN. To
guarantee that the ripple current stays below the
specified maximum, the inductor value should be
chosen according to the following equation :
OUT OUT
L(MAX) IN(MAX)
V VL 1
f I V
RT7296B
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The inductor's current rating (caused a 40C
temperature rising from 25C ambient) should be
greater than the maximum load current and its
saturation current should be greater than the short
circuit peak current limit.
CIN and COUT Selection
The input capacitance, CIN, is needed to filter the
trapezoidal current at the source of the top MOSFET.
To prevent large ripple current, a low ESR input
capacitor sized for the maximum RMS current should
be used. The RMS current is given by :
OUT INRMS OUT(MAX)
IN OUT
V VI I 1
V V
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is
commonly used for design because even significant
deviations do not offer much relief.
Choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet size or height requirements in the design. The
selection of COUT is determined by the required
Effective Series Resistance (ESR) to minimize voltage
ripple. Moreover, the amount of bulk capacitance is
also a key for COUT selection to ensure that the control
loop is stable. Loop stability can be checked by viewing
the load transient response as described in a later
section. The output ripple, VOUT, is determined by :
OUT LOUT
1V I ESR
8fC
The output ripple will be highest at the maximum input
voltage since IL increases with input voltage. Multiple
capacitors placed in parallel may be needed to meet
the ESR and RMS current handling requirement. Dry
tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount
packages. Special polymer capacitors offer very low
ESR value. However, it provides lower capacitance
density than other types. Although Tantalum capacitors
have the highest capacitance density, it is important to
only use types that pass the surge test for use in
switching power supplies. Aluminum electrolytic
capacitors have significantly higher ESR. However, it
can be used in cost-sensitive applications for ripple
current rating and long term reliability considerations.
Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient
and audible piezoelectric effects. The high Q of
ceramic capacitors with trace inductance can also lead
to significant ringing.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature.
The maximum power dissipation can be calculated by
the following formula :
PD(MAX) = (TJ(MAX) TA) / JA
where TJ(MAX) is the maximum junction temperature,
TA is the ambient temperature, and JA is the junction to
ambient thermal resistance.
For recommended operating condition specifications,
the maximum junction temperature is 125C. The
junction to ambient thermal resistance, JA, is layout
dependent. For TSOT-23-8 (FC) package, the thermal
resistance, JA, is 70C/W on a standard JEDEC 51-7
four-layer thermal test board. The maximum power
dissipation at TA = 25C can be calculated by the
following formula :
PD(MAX) = (125C 25C) / (70C/W) = 1.428W for
TSOT-23-8 (FC) package
The maximum power dissipation depends on the
operating ambient temperature for fixed TJ(MAX) and
thermal resistance, JA. The derating curve in Figure 5
allows the designer to see the effect of rising ambient
temperature on the maximum power dissipation.
RT7296B
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Figure 5. Derating Curve of Maximum Power
Dissipation
Layout Considerations
For best performance of the RT7296B, the following
layout guidelines must be strictly followed.
Input capacitor must be placed as close to the IC as
possible.
SW should be connected to inductor by wide and
short trace. Keep sensitive components away from
this trace.
Keep VIN, GND and SW connected to pin as wide as
possible for improving thermal dissipation.
SW
VOUT
R1
R2
CIN
CIN COUT COUT
Keep the SW trace as physically short and wide as
practical to minimize radiated emissions and enables
better thermal performance.
Via can help to reduce
power trace and improve
thermal dissipation.The feedback components
must be connected as close
to the device as possible.
VOUTGND
SS
SW
FB
EN/SYNC
BOOT
VINPVCC
5
34
68
27
GNDCss
Input capacitor must be placed as close
to the IC as possible. Suggestion layout
trace wider for thermal.
R5
Figure 6. PCB Layout Guide
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0 25 50 75 100 125
Ambient Temperature (°C)
Ma
xim
um
Po
we
r D
issip
atio
n (
W) 1 Four-Layer PCB
RT7296B
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Outline Dimension
Symbol Dimensions In Millimeters Dimensions In Inches
Min. Max. Min. Max.
A 0.700 1.000 0.028 0.039
A1 0.000 0.100 0.000 0.004
B 1.397 1.803 0.055 0.071
b 0.220 0.380 0.009 0.015
C 2.591 3.000 0.102 0.118
D 2.692 3.099 0.106 0.122
e 0.585 0.715 0.023 0.028
H 0.080 0.254 0.003 0.010
L 0.300 0.610 0.012 0.024
TSOT-23-8 (FC) Surface Mount Package
Richtek Technology Corporation
14F, No. 8, Tai Yuen 1st Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863)5526789 Richtek products are sold by description only. Customers should obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.